The present invention relates to a process of fabricating a gradient index type of optical element (hereinafter often referred to as a gradient index optical element) applicable to optical elements for cameras, microscopes, endoscopes, electronic imaging systems, etc.
A gradient index optical element includes a medium to which a gradient index is so imparted that the medium itself can have (refracting) power. Gradient index optical elements now draw attention as optical elements inevitable for optical systems of the next generation because their excellent ability to compensate for aberrations will make it possible to reduce the number of lenses which optical systems have.
Depending on diametrical changes in refractive index and Abbe number, the gradient index optical element is broken down into two types: one called a positive refracting power type of gradient index optical element having refracting power decreasing from a central direction to a peripheral portion of glass, and the other called a negative refracting power type of gradient index optical element of increasing refracting power, as viewed in the same direction. An element which has an Abbe number that decreases with an increase of refractive index is called a high-dispersion profile type, an element which has an Abbe number that increases rather than decreases called a negative dispersion profile type, and an element which has an Abbe number that remains virtually constant is called a low-dispersion profile type. Various combinations of refractive index changes and Abbe number changes lead to optical elements which include an element called a positive refracting power type of negative dispersion gradient index optical element, and an element called an ultra-dispersion gradient index optical element in which its refractive index does not much change but its Abbe number changes by a large amount.
"Microoptics News", Vol. 9-3 (1991), pp. 13-18, Japan Optics, '92 Preprint, JP-A-3-141302, "SPIE", Vol. 1780 (1992), pp. 456-463 and other publication disclose that the low-dispersion, and negative dispersion types are effective for gradient index optical elements which are usable with white light sources, and are excellent in the ability to make correction for chromatic aberration. "Applied Optics", Vol. 25, No. 18 (1986), pp. 3351-3355, too, reports that the negative refractive power type of gradient index optical element excels in its ability to correct for aberrations.
On the other hand, JP-A-6-148405 discloses an ultra-dispersion gradient index composition. To fabricate such a gradient index optical element with excellent ability to correct for chromatic aberration, the concentration of a metal component such as Ti, Nb, and Ta must have a so-called concave profile such that it increases continuously from a central direction to a peripheral portion of glass.
In general, the gradient index optical element is fabricated by sol-gel, ion exchange, molecular stuffing, and other techniques. In particular, the sol-gel method now attracts attention for reasons of some characteristic advantages. For example, this makes it possible to obtain glasses having a large aperture, and enables oxides of polyvalent metals to have a profile and the resulting gradient index optical element to vary in optical characteristics.
JP-A-4-108626 discloses a process of imparting a diametrically concave concentration profile of reaction solution to a dopant-containing wet gel by cooling the system to a temperature at which the wet gel does not react with the reaction solution. Problems with this process are, however, that the solubility of metal components varies largely with a temperature change of the solution, resulting in the precipitation of the metal components, difficulty is encountered in the selection of the solvent to be used, and the reproducibility of the refractive index profile obtained is less than satisfactory.
JP-A-5-306126 discloses a process of introducing a metal salt into a wet gel from outside to obtain a negative refracting power type of gradient index optical element. However, a gradient index optical element such as one, to which metal components contributing greatly to a refractive index, for instance, Ti, Nb, Ta, and Zr are imparted in the form of a diametrically convex profile, has difficulty in applications because no suitable salts of those metals are available.
JP-A-4-55339 discloses that first dopant-containing gels are immersed in a suitable eluting solution to impart a profile to the first dopant, and to give a profile to a second component as well. When attempts are made at obtaining gradient index glasses having low dispersion or a negative dispersion profile, on the other hand, it is required to introduce metal species to be convexly distributed in wet gels, for instance, Ba, La, Y, Gd, Sr, Ca, Ge, Zr, and Zn, in the wet gels in the form of metal alkoxides. However, some limitation is imposed on the amount of the alkoxides of these metal species introduced in the wet gels because most of them are solid or low in solubility. In other words, no glasses having a large concentration difference are obtainable because it is impossible to impart a large metal concentration difference to the wet gels.
JP-A-62-292624, and JP-B-5-82332 disclose processes of obtaining glasses containing metals such as SiO.sub.2 --TiO.sub.2 by drying wet gels into dry gels, firing the dry gels into a porous body, and dipping the porous body in a solution containing Ti or the like to homogeneously impregnate the body with the metal component. However, when these processes are used to attempt imparting a diametrically concave profile to titanium by reducing the dipping time of the porous body in the titanium-containing solution, some problems arise. That is, since the dry gels have been thermally treated at high temperature, Si--O--Si bond arms are firmly bonded with one another or, in another parlance, the number of reactive Si--OH groups is reduced, resulting in too little a titanium component being bonded to Si--O--Si. Consequently, portions of the titanium component which take no part in Si--O--Ti bonds aggregate upon being fired, and so cause crystallization of anatase responsible for devitrification. In addition, the optical properties of lenses obtained by polishing the resulting glasses become undesirable due to titanium inhomogeneity in minute regions.
JP-B-7-33248 discloses a process of modifying the surface of a so-called silica aerogel exhibiting various unique physical properties, for instance, having transparency, thermal insulation, low density, and low refractive index by dipping silica gels obtained from sols hydrolyzed by the addition of ammonia thereto in an alcohol, using a dehydrating agent to dehydrate the alcohol including a liquid phase in the wet gels, diffusing an alcohol solution of a metal oxide capable of condensing with a surface hydroxyl group of silica into voids in a network structure of wet gels for condensation with the surface hydroxyl group of silica, removing unreacted metal compounds from the wet gels, and finally subjecting the wet gels to ultra-critical drying.
Even when the silica aerogel is thermally treated for surface modification purposes, however, it is heated at barely about 500.degree. C. at which pores or voids remain intact. Accordingly, growth of titanium oxide nuclei are not so noticeable that the physical properties are little, if any, affected. In the present invention, however, it is required that gels be heated at such a high temperature at which pores vanish due to vitrification. In the process during which the gels are heated to the temperature at which they are deprived of pores, there has been observed a phenomenon that the crystallization of the whole system proceeds with the nucleation of titanium oxide, thus causing anatase to be crystallized out in the resulting glass and, hence, making the glass whitish and crystalline.
Processes for treating sols with salts of metals such as barium or lanthanum previously contained in them often cause the precipitation of the metal salts when the pH of the sols is shifted to an alkaline side, thus failing to obtain homogeneous sols. For these processes, therefore, it is required that the pH of the sols be shifted to an acidic side by the addition of acetic or other acid thereto.
JP-A-61-101425 discloses a process of obtaining gradient index glasses by dipping wet gels containing both silicon alkoxide and a refractive index-increasing metal alkoxide component in a solution containing a refractive index-lowering metal alkoxide for exchange reactions between metal species. With this process, however, it is impossible to fabricate gradient index lenses having a low or negative dispersion profile, or an ultra-dispersion profile, because of difficulty in preparing alkoxides of stable yet easy-to-handle metals such as Ba, and so the metal species selected for the low refractive index component is practically limited to phosphorus, boron, and the like.
A primary object of the present invention is to provide a process of fabricating a gradient index optical element by the sol-gel method, and a particular object of the present invention is to provide a process of fabricating a gradient index type of optical element--which has a refractive index profile controlled with high precision and excels in the ability to correct for chromatic aberration--at lesser steps, in higher yields, and in more rapid manners, when compared with conventional processes, in which process the concentration of a metal component which is unavailable in the form of a suitable salt capable of being introduced in a wet gel, for instance, Ti, Nb, Ta, and zr is provided in a diametrically concave profile, and the concentration of a metal component selected from Ba, La, Y, Gd, Sr, Ca, Ge, Zr, and Zn is provided in a diametrically convex profile.